Optical device producing an intensity dependent phase shift

ABSTRACT

An optical device comprises an optical waveguide (2) formed from at least a first material having a non-linear refractive index n 2  coupled to a first pair of ports (4,5) of an optical coupler (3). An optical signal input at one of the second pair of ports (6,7) of the coupler (3) is split to provide two signals counter propagating around the waveguide loop (2). By selecting the coupling ratio and appropriate waveguide parameters to ensure an asymmetry in the device it is possible to produce an intensity dependent relative phase shift between the counter propagating signals, thereby to vary the device output. Embodiments of the invention may be used to perform logic functions on, to amplify, switch or otherwise modify an input signal.

FIELD OF THE INVENTION

The invention relates to an optical device for use for example as anoptical amplifier or logic element.

BACKGROUND AND SUMMARY OF THE INVENTION

A paper entitled "Nonlinear antiresonant ring interferometer" in OpticsLetters, Vol. 8, No. 9, pages 471-473, by Kenju Otsuka, describes aninterferometer comprising a beam splitter to split an optical beam intoportions of different intensities, a pair of mirrors on to respectiveones of which the portions impinge, and a block of non-linear materialpositioned in the optical path between the mirrors. Light is split atthe beam splitter into the different intensity portions which are thencaused to pass in opposite directions through the non-linear mediumwhich has the effect of imparting different phase shifts on the lightportions due to its non-linear refractive index property. The phaseshifted portions are recombined at the beam splitter.

The Otsuka device relies on cross-interaction between the two counterpropagating optical fields and is dependent on interference between thefields producing a non-linear index grating within the non-linearmedium. In these circumstances the counter propagating fields in thedevice are necessarily of a duration which exceeds the propagationperiod within the non-linear medium. The device operation requires theoptical fields to be coincident in the non-linear medium therebynecessitating precise location of the non-linear medium at the mid-pointof the optical path around the device. One of the problems with thisdevice is therefore the need for accurate positioning of the variouscomponents. For example, the mirrors also have to be very accuratelyaligned with the non-linear medium and with each other.

In addition, to avoid problems which would arise from field divergencewithin an extended non-guiding medium, the length of the non-linearmedium itself is restricted. There is a further risk of diffractionproblems because the optical fields are laterally unconstrained duringpropagation around the device i.e.. the fields could spread outlaterally which would reduce their intensities.

It is an object of the present invention to provide an optical devicewhich substantially overcomes or at least mitigates the aforementionedproblems and restrictions. It is a further object of the presentinvention to provide a method of operation of such a device.

In a first aspect the present invention provides an optical devicecomprising a coupling means having first and second pairs of opticalcommunication ports, in which portions of an optical signal received ata port of one pair are coupled into each port of the other pair in apredetermined coupling ratio; and an optical waveguide at least aportion of which includes a first material having a non-linearrefractive index, the optical waveguide coupling together the first pairof ports; the coupling ratio and appropriate waveguide parameters beingselected such that in use the portions of an optical signal at a workingintensity received at one of the second pair of ports of the couplingmeans and coupled into each end of the waveguide return with anintensity dependent relative phase shift after travelling around thewaveguide.

In contrast to the known device described above, the present inventionmakes use of an optical waveguide including a material having anon-linear refractive index. This not only enables previouslyencountered alignment and diffraction problems to be avoided, butfurthermore provides more flexibility in operation and avoids theconstructional limitations of the earlier device. Further, the presentdevice does not require cross-interaction between counter propagatingfields, nor the establishment of an interference grating. Thus, incontrast to the device of Otsuka, the present device enables anintensity dependent relative phase shift to be achieved where theduration of an input signal is shorter than the signal transit timethrough the non-linear medium or material. The non-linear material mayalso be conveniently distributed throughout the waveguide.

In the present device the appropriate waveguide parameters, the couplingratio, or a combination of both may be selected to break the devicesymmetry and so obtain a relative phase shift in the counter propagatingportions as will be further described below. Thus, for example, thecoupling ratio may be symmetric (50:50), in contrast to the Otsukadevice where it is essential for the beamsplitter to be assymmetric(i.e. other than 50:50) as the optical path in his device is otherwisesymmetric.

The waveguide parameters which may be appropriately selected to affectthe device symmetry include, for example, the waveguide length, thenon-linear refractive index n₂ (Kerr co-efficient), the dispersion k₂,the mode field width, and the like. These parameters may be allowed tovary along the length of the waveguide.

In this specification by "non-linear" we mean that the refractive indexof the material varies with the intensity of the transmitted signal.Typically, the refractive index n is given by the formula:

    n=n.sub.0 +n.sub.2 |E|.sup.2

where n₀ is the linear refractive index, n₂ is the Kerr co-efficient and|E|² is the intensity of the transmitted signal.

In one preferred arrangement the coupling means has a coupling ratio ofother than 50:50 (i.e.. the intensities of signal portions coupled intothe ends of the waveguide are not equal). In this situation, signalswith different intensities are fed in opposite directions around thewaveguide thus resulting in the signals experiencing differentrefractive indices. As will be explained below, this results in thesignals experiencing different phase shifts so that when the signalsreturn back to the coupling means, they have an intensity dependentrelative phase shift. By varying the coupling ratio and/or the length ofthe waveguide, for example, it is possible to vary the phase shiftbetween the returning signals for any particular working intensity ofinput signal.

The intensity dependence of the relative phase shift results in a devicewhose output is an oscillatory function of the intensity of the inputsignal. This property can be used in a variety of applications includinglogic elements, optical amplifiers, optical switches and the like.

In another arrangement, the waveguide may further comprise a secondmaterial in series with the first material, the first and secondmaterials having non-commuting effects on an optical signal at a workingintensity travelling along the waveguide. In this situation, thecoupling ratio of the coupling means could be 50:50 since thenon-commuting materials can be arranged to automatically produce therequired relative phase shift even in signal portions with the sameinput intensity. The second material is preferably a dispersivematerial. Conventionally, it is desirable to minimise dispersioneffects, both by fabricating waveguides with low absolute dispersion andby operating at wavelengths around the dispersion zero for thewaveguide. However, a waveguide according to the present invention canbe fabricated with different dispersive properties at differentportions. For example, differences in total dispersion can be achievedby varying the waveguide refractive index profile. According to thelength of the waveguide portion comprising the second material, thedispersion must be adequate, in combination with an appropriatenon-commuting property, to provide the asymmetry required to achieve theintensity dependent phase shift. Suitable combinations of non-commutingproperties include, for example, dispersion and either (or both) ofnon-linearity n₂ and mode field width. Alternatively, for example, thesecond material may have non-linear polarisation rotation propertieswhich do not commute with those mentioned above. Appropriate alternativecombinations of these and other properties will be apparent to thoseskilled in the art.

It should be noted that where the waveguide comprises two or moreserially connected portions with non-commuting properties then the orderin which non-interacting, counter propagating signals pass through theportions becomes important and changing the order will generally resultin a different phase change in the resultant signals arriving back atthe coupling means.

Devices according to the invention are operable to produce an intensitydependent phase shift both when the duration of the counter propagatingsignals exceeds the transit time through the waveguide (whencross-interaction dominates) and when the signal duration is less thanthis transit time (when the cross-interaction is not significant).However, the operation of the device as discussed above assumes that theinput signals are of substantially constant intensity over the timetaken to propagate around the waveguide. For pulse signals this amountsto an assumption that the pulses are substantially square. As pulseduration decreases, however, this assumption is no longer valid for realpulses with finite rise and fall times which comprise a significantproportion of the overall pulse width. In these circumstances each pulseenvelope will contain a number of cycles with a range of intensities. Insilica, for example, since the non-linear refractive index responds tothe instantaneous intensity, each cycle will experience a slightlydifferent refractive index as it passes through the non-linear materialwhich will generally result in a variation in phase shift between cyclesin the same pulse which may degrade the basic device performance.

In a preferred embodiment of the present invention, to overcome or atleast mitigate the potential problem which may be presented under theseconditions, the waveguide comprises material which supports solitoneffects when optical pulses at appropriate working intensities areinjected into the device. The length of the waveguide must then besufficient such that the intensity dependent phase of an injected pulsebecomes substantially uniform throughout the pulse.

In this latter embodiment the properties of the waveguide are selectedsuch that the Kerr coefficient, n₂, and the group velocity dispersionhave opposite signs. Then, if the input is of sufficiently highintensity, the waveguide will support pulses which propagatesubstantially non-dispersively over several times the length over whicha low intensity pulse would disperse. Such pulses are referred to assolitons. An article by N. J. Doran and K. J. Blow entitled "Solitons inOptical Communications", IEEE Journal of Quantum Electronics, Vol. QE19,No. 12, Dec. 1983 provides an appropriate discussion of solitonpropagation. In the present specification and relevant claims "soliton"is taken to refer to any pulse which exhibits the above property ofsubstantially non-dispersive propagation and not only to so-called"exact" or pure solitons, for example, as hereinafter described.

This preferred embodiment, therefore, specifically employs a waveguidewith significant dispersion of the required form which permits solitonpropagation.

For soliton pulses the overall phase changes are dependent on theintensity of the pulse envelope as a whole and not merely on theinstantaneous intensities of different portions of the wave train as isthe case with non-soliton pulses. For the intensity-dependent phase of asoliton pulse to be substantially uniform throughout the pulse, it hasbeen found that solitons should propagate over a waveguide length atleast approximately equivalent to a soliton period or more as describedbelow.

As with the previous embodiments of the invention, to achieve anon-zero, intensity-dependent relative phase shift between the wavetrains within the counter propagating pulse envelopes it is necessary tobreak the symmetry of the device in some appropriate manner.Conveniently, this may be done by using an asymmetric coupling means(not 50:50) or by having waveguide portions with different dispersionsor non-linear coefficients n₂, for example. However, since therefractive index varies with n₂ ×Intensity, an effective asymmetry mayalso be obtained by allowing the intensity in different portions of thewaveguide to differ. This may be achieved, for example, by havingdifferent portions of the waveguide with differing mode field widths.Any combinations of these differences may also be used to achieve adesired asymmetry.

For soliton propagation, the waveguide preferably comprises materialwhich simultaneously exhibits both the dispersive and non-linearproperties as required for soliton propagation. Whilst it is possible toachieve soliton propagation under alternative conditions, for example,when the waveguide comprises an alternating sequence of dispersive andnon-linear components, this is not particularly desirable for solitonpropagation since, as noted above, in physical terms, the effects do notcommute. Consequently, a large number of very short lengths of waveguidewith the alternating properties would probably be required to achieve areasonable approximation to the conditions for effective solitonpropagation.

Preferably, the waveguide is a single mode waveguide. Conveniently, theoptical waveguide is formed from optical fibre, preferably monomodeoptical fibre. Alternatively, for example, the waveguide may befabricated in planar (e.g. lithium niobate) waveguide form.

Non-linear properties may be provided by appropriately doping thewaveguide. It is also possible, for example, to introduce non-linearbehaviour by providing suitable non-linear material as an overlay on aconventional waveguide. For instance, an optical fibre may have somecladding etched away sufficiently to allow coupling of its optical fieldinto an external overlay of non-linear material without necessarilyexposing or doping the fibre core. Similarly the dispersive propertiesmay be provided by doping or other techniques. For example, a dispersivegrating may be provided in the waveguide.

Also preferably the waveguide includes inherent polarisation control oris positioned in series with a polarisation controller. Where thewaveguide exhibits birefringence, for example, polarisation controlenables the input to be appropriately adjusted or maintained to provideconsistent and predictable device performance.

According to another aspect of the present invention a method ofprocessing an optical signal comprises the steps of:

providing a device according to the invention in its first aspect;

inputting an optical signal into a second port of the device to producetwo counter propagating signals within the waveguide, thereby to providea processed pulse signal output at least at one of the second pair ofports.

Preferably, the method comprises inputting an optical signal having aduration less than the transit time for propagation around the waveguideand of substantially constant input intensity.

The processing may be to perform logic functions on, to amplify, switchor otherwise modify an input signal. The processed signal output willcorrespondingly comprise a logical output, an amplified, switched orotherwise modified signal. Criteria and preferences for the selection ofdevice parameters are as described above with reference to the inventionin its first aspect.

Alternatively, or additionally, for processing an optical signalcomprising pulses capable of soliton propagation the method comprisesthe steps of:

providing a device according to the invention in its first aspectincluding a waveguide whose parameters are selected for solitonpropagation;

inputting a pulse signal into a second port of the device, at anamplitude appropriate for soliton propagation in the waveguide, therebyto produce two counter propagating signals within the waveguide and toprovide a processed pulse signal output at least at one of the secondpair of ports.

The device parameters are selected appropriately to influence thesoliton propagation according to the processing required.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of an optical device and methods of operation according tothe invention will now be described by way of example and with referenceto the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a device;

FIG. 2 illustrates graphically the variation between input intensity andoutput intensity (after normalisation) for two different couplingratios;

FIG. 3 is a graph illustrating the propagation characteristics of adevice according to the invention for a pulse input;

FIG. 4 illustrates a series of devices coupled together; and

FIGS. 5 and 6 show devices according to the invention in furtherconfigurations.

DETAILED DESCRIPTION

The optical device shown in FIG. 1 is defined by a single silica opticalfibre 1 formed into a loop 2. Portions of the optical fibre itself arepositioned close to one another to define an X coupler 3 having a firstpair of ports 4, 5 and a second pair of ports 6, 7. The X coupler 3 isadapted to couple portions of an optical signal received at the ports 6or 7 into each of the ports 4, 5 in accordance with a predeterminedcoupling ratio. Similarly, optical signals arriving at the coupler 3received by either the port 4 or port 5 will be coupled by the samecoupling ratio into the ports 6, 7.

Thus, under normal (linear) circumstances the device shown in FIG. 1will operate as a mirror. That is with the input E₁ at the port 6, thelight all returns back to the port 6 if the coupling ratio is 50:50. Anyother value for the coupling ratio gives an appropriate split in theoutput power between the two ports 6, 7.

The coupler 3 causes a single input to be split into two counterpropagating fields which return in coincidence to combine back at thecoupler. The optical path length is precisely the same for bothpropagating fields since they follow the same path but in oppositedirections. This is an important feature of the device. Linearly thefields will emerge the same independent of which way round they traversethe loop; non-linearly this may not be the case. Below, a situation isdescribed in which non-linearity breaks the identical propagationeffects for the two paths. The device described responds to differencesin phases of combining fields and could therefore be described as aninterferometric device but there is no need for interferometricalignment of the optical paths. This is an outstanding feature of thedevice.

The loop 2 of the waveguide is formed at least partly from a non-linearmaterial exhibiting a refractive index n which varies in accordance withthe formula given above. The device operates in the `small`non-linearity regime which means that it is only the phase that isaltered and there are, for example, no effects on the group velocity. Inthe small non-linearity regime, n₀ is much greater than n₂ |E|² (e.g.may be by a factor of about 10⁶). The simplest realization of anon-linear device based on the configuration of FIG. 1 is to allowself-phase-modulation (SPM) in the fibre loop with a coupler 3 withcoupling ratio α:(1-α)

SPM gives a distance and intensity dependent phase shift given by

    φ(E)=(2πn.sub.2 |E|.sup.2 L)/λrad

where n₂ is the nonlinear Kerr coefficient.

The coupler equations for an `X` coupler as in FIG. 1 are:

    E.sub.3 =α.sup.0.5 E.sub.1 +i(1-α).sup.0.5 E.sub.2 (1)

    E.sub.4 =i(1-α).sup.0.5 E.sub.1 +α.sup.0.5 E.sub.2 (2)

with α the coupling coefficient (e.g. for a 50:50 coupler α=0.5, for a60:40 coupler α=0.4).

We take the case of a single input at port 6, E_(IN). Therefore:

    E.sub.3 =α.sup.0.5 E.sub.IN

    E.sub.4 =i(1-α).sup.0.5 E.sub.IN

Thus after travelling the distance L

    E.sub.3 =α.sup.0.5 E.sub.IN expi[α2πn.sub.2 |E.sub.IN |.sup.2 L/λ.sub.2 ]

    E.sub.4 =i(1-α).sup.0.5 E.sub.IN expi[(1-α)2πn.sub.2 |E.sub.IN |.sup.2 L/λ]

For the return transit through the coupler 3 we need the transform ofequation 1, i.e.:

    E.sub.1 =α.sup.0.5 E.sub.3 -i(131 α).sup.0.5 E.sub.4

    E.sub.2 =i(1-α).sup.0.5 E.sub.3 +α.sup.0.5 E.sub.4

To calculate the output at 6 and 7 we need to substitute and E₄ =E₃ *and E₃ =E₄ * and obtain:

    E.sub.1 =-iα.sup.0.5 (1-α).sup.0.5 E.sub.IN [exp-i((1-α)φ(E.sub.IN)))+exp-i(αφ(E.sub.IN))]

    E.sub.2 E.sub.IN [αexp-i(αφ(E.sub.IN))-1-α)exp-i((1-α)φ(E.sub.IN))]

The output intensities are given by:

    |E.sub.1 |.sup.2 =|E.sub.IN |.sup.2 2α(1-α)[1+cos((1-2α)φ(E.sub.IN))]

    |E.sub.2 |.sup.2 =|E.sub.IN |.sup.2 [1-2α(1-α)(1+cos((1-2αφ(E.sub.IN))]

These equations show that for any value of α, 100% of the power emergesfrom port 7 when:

    φ(E)=π(1-2α)m

and the minimum output power from port 7 is when:

    φ(E)=2π(1-2α)m,

where m is an integer; in which case:

    |E.sub.2 |.sup.2 =1-4α(1-α)

which is the output for linear fields.

The general behaviour (or response characteristic) is shown in FIG. 2.The output switches from the low power value to 100% every time thepower increases by ##EQU1## The best switching ratio occurs for α closeto 0.5 but the switching energy increases correspondingly. In the limit,for a 50:50 coupler (α=0.5) the required field would be infinite.

For one shift from minimum to maximum output we require: ##EQU2##

For silica based fibres n₂ =3.2×10⁻¹⁶ cm² /Wm, and taking λ=106 μm andfibre area 100 μm² then: ##EQU3##

For the example of α=0.4 then we need E² =8 kw for L=1 m. If α=0.1 thenthe required peak power comes down to 1.9 kw but the switching contrastis correspondingly reduced as shown in FIG. 2.

The above calculations are effectively for constant intensity operationand do not treat the case where dispersion is significant. A singlevalue has been taken for the input intensity in order to derive theoutput results. In reality, unless the input can be considered as squarepulses, the transmission characteristic will be degraded by the varyingsignal intensity. Since the basic device response is just to theinstantaneous intensity, the basic device would not work as efficientlyon shorter pulses where the variation in intensity throughout the pulseduration becomes significant.

It is then desirable to modify the device to provide significantdispersion as well as SPM in the waveguide and introduce solitonpropagation effects.

Solitons are generated by the combined action of self-phase modulationand dispersion in the negative group velocity dispersion regime in anoptical waveguide. The use of solitons in a non-linear optical device isalso discussed in copending patent application GB No. 8625281 filed22.10.86 in the name of the present applicants.

An exact single soliton does not change shape in its propagation throughan optical waveguide, but it does acquire a phase shift proportional tothe distance travelled. Even if the pulse is not an exact soliton theeffects of dispersion and non-linearity can be approximately balanced,and a pulse whose amplitude and shape is close to an exact soliton doesnot change significantly on propagation.

The propagation of pulse envelopes u(z,t) in a waveguide with negativegroup velocity dispersion and including non-linearity is described bythe dimensionless Non-linear Schrodinger equation (NLS):

    iu.sub.z +U.sub.tt /2+u|u|.sup.2 =0

where the subscripts imply partial differentials. The requirement fornegative group velocity dispersion determines the positive sign of the|u|² term in the NLS. This is a normalised equation and there aretransformations to convert the dimensionless quantities back to realunits (see e.g. Doran and Blow op. cit.). Here it is sufficient to pointout that the normalised amplitude generated by a real pulse isproportional to (n₂ /k₂)^(1/2) and the normalised distance, z isproportional to k₂ /T² L, where k₂ is the dispersion coefficient, T isthe pulse duration and L is the real distance. The NLS has exact solitonsolutions of the initial form

    u(z=0,t)=Nsech(t)

with N integer. For all N(integer) the solitons have the property thatthe modulus of u (and thus the shape of the pulse envelope) returns toits original form every π/2 propagated (i.e. the soliton period is π/2).For N=1 the full solution is

    u(z,t)=exp(iz/2)sech(t)

It is important to note in the above formula the phase factor exp(iz/2).This is an overall phase which is present in all solitons. That is forall solitons the solution can be written

    u(z,t)=exp(iz/2)f(z,t)

where f(z,t) is periodic in z with period π/2. It is this property ofsolitons which can be exploited in an embodiment of the presentinvention adapted to allow soliton propagation.

From numerical solutions of the propagation problem the presentinventors have found that pulses in the soliton regime but whoseamplitudes do not correspond to that of exact solitons acquire anoverall phase shift proportional to the distance travelled. This phaserotation is approximately uniform throughout the intensity envelope, andincreases with peak amplitude. If the pulse amplitude in a deviceaccording to the invention is sufficient to produce these `soliton`effects, then good switching is still possible for entire pulses.

For a given dispersion, the length of the waveguide must then besufficient to provide for communication between the different intensitycycles within a pulse, such that the intensity dependent phase of aninjected pulse becomes substantially uniform throughout the pulse.

As a soliton pulse propagates in a waveguide, cycles of the wave trainwithin the pulse envelope which defines the soliton undergointensity-dependent phase changes. After some distance of propagationthe intensity-dependent phase is essentially uniform throughout the wavetrain forming the soliton. Thus overall phase changes are dependent onthe intensity of the pulse envelope as a whole and not merely on theinstantaneous intensities of different portions of the wave train as isthe case with non-soliton pulses. For the intensity-dependent phase of asoliton pulse to be substantially uniform throughout the pulse it hasbeen found that solitons should propagate over a waveguide length atleast approximately equivalent to a soliton period or more.

FIG. 3 illustrates as an example the device characteristics for awaveguide formed of a loop of fibre with a length equivalent to foursoliton periods. For a standard fibre at 1.55 μm, this is equal to about100 m for a 1 ps pulse. The total output energy is shown as a functionof input energy for a sech shaped input pulse. In these circumstances itcan be seen that the switching characteristics are comparable to thoseillustrated in FIG. 2, but in this case entire pulses are switched.

For comparison, FIG. 3 also shows the result for the same input pulsesfor a device with a waveguide with insignificant dispersion such thatthere is no substantial interplay between dispersion and the non-linearrefractive index to allow soliton propagation. Using a waveguide adaptedto allow soliton effects, there is a very clear improvement inperformance. The units of FIG. 3 are given in terms of the energy of asingle soliton; the conversion to real energies depends on the assumedpulse duration. Typically, for a standard (i.e. not dispersion shifted)optical fibre with say an effective area of 30 μm² and taking n₂=3.2×10⁻¹⁶ cm² /W, at λ=1.55 μm, then a 7 ps soliton will have an energyof around 2 pJ. For a 1 ps soliton this increases to around 15 pJ with acorresponding reduction required in the loop length.

Thus if the device is adapted to operate in the soliton regime, thenexcellent switching characteristics can be obtained for entire`bell-shaped` pulses. Generally, it will be necessary for the loop to beof sufficient length for dispersion to take effect, which in practicemeans around one or more soliton periods. The loop length actuallyrequired reduces as the square of the pulse duration. Thus forsubpicosecond switching, a loop of only a few meters of fibre would berequired.

The device may also be fabricated in planar (e.g. LiNbO₃) waveguideform. The appropriate dispersion effects may be obtained, for example,by means of imposing a grating in the basic loop.

It should be noted that in devices here dispersion is not significant(e.g. where soliton propagation is not especially desirable), there isno necessity for the non-linear material to be evenly distributedthroughout the waveguide forming the loop. The device operation is thensubstantially insensitive to the positioning of the non-linearity withinthe loop, and therefore equivalent effects can be obtained, for example,by inserting a short, more highly non-linear element anywhere in a loopof otherwise standard optical fibre.

The device described above may be concatenated as shown in FIG. 4 toimprove the switching contrast. FIG. 4 illustrates two devices 8, 9coupled together in series. Unlike alternative Mach-Zehnder baseddevices there is no need to arrange for phase shift similarity forconsecutive elements in the concatenation since there is no phase shiftparameter. Interferometric alignment is guaranteed in the presentdevices.

The symmetry can be broken for the 50:50 coupler situation without theneed for infinite fields as is the case in the simple device describedabove. To do this, for example, it is necessary to have at least twotypes of fibre arranged in series within the loop 2 of FIG. 1. Ingeneral, where there is non-linearity, propagation in a first type offibre followed by propagation in the second type will not result in thesame output as propagation in the second type of fibre followed bypropagation in the first. This is the principle to be exploited. Forexample, the first type of fibre could be selected to have a dispersionzero at the operating wavelength, in which case the propagation would beby SPM, whilst the second fibre type could be selected to be highlydispersive at the operating wavelength such that propagation would besubstantially pure dispersion. These two effects do not commute.

Symmetry breaking may generally be expected to require dispersioneffects and therefore this type of configuration is appropriate foroptimisation of logic operation for pulses. The device could operate inthe soliton regime and give good switching for whole pulses without theneed for interferometric alignment. Other combinations of non-commutingeffects including non-linear polarisation rotation and mode field width,for example, may also be used.

The basic device shown in FIG. 1 can be utilised in a number ofapplications. For example, the device can be fabricated as an amplifierby biasing the input port 6 to a position towards the bottom of one ofthe curves shown in FIG. 2. As shown in FIG. 5, this may be achieved bythe addition of a Y coupler 10 to the basic device. One input arm of theY coupler 10 is coupled with a laser 11 which generates a bias opticalsignal E_(BIAS) and the other input arm 12 is coupled with a source ofoptical signals E_(S). If the device is biased at a position near thebase of a steeply sloping portion of the appropriate characteristiccurve then a small input signal E_(S) fed along the other arm 12 of theY coupler will cause a signal with a significantly increased intensityto be output from the port 7.

The device can also be used as a logic element, for example an EXORgate. A simple two input EXOR gate configuration is shown in FIG. 6. Inthis case, a Y coupler 10 is again provided with its output coupled withthe port 6 and with both its input arms coupled with respective signalsources E_(A) and E_(B). With the input logic levels selected such thata LOW input coincides with an intensity E_(IN) corresponding to aminimum in the response characteristic (FIG. 2. and with the differencebetween a LOW and a HIGH chosen to provide a change in intensitysufficient to move to a maximum in the response characteristic, a highoutput E_(OUT) at the port 7 will only be generated when the one or theother, but not both, of the two input signals is HIGH.

Other configurations employing embodiments of the present invention willbe apparent to those skilled in the art. For example, a suitablycalibrated device according to the invention may be used for thedetermination of the unknown n₂ of a material inserted in the waveguideloop by measuring the phase shift for a given intensity input.

What is claimed:
 1. An optical device comprising a coupling means havingfirst and second pairs of optical communication ports, in which portionsof an optical signal received at a port of one pair are coupled intoeach port of the other pair in a predetermined coupling ratio; and anoptical waveguide including a first material having a non-linearrefractive index, and a second material in series with the firstmaterial, the first and second materials having non-commuting effects onan optical signal at a working intensity travelling along the waveguide,the optical waveguide coupling together the first pair of ports; thecoupling ratio and appropriate waveguide parameters being selected suchthat in use the portions of an optical signal at a working intensityreceived at one of the second pair of ports of the coupling means, andcoupled into each end of the waveguide, return to said coupling meanswith an intensity dependent relative phase shift after travelling aroundthe waveguide.
 2. A device according to claim 1, wherein the couplingratio of the coupling means is other than 50:50.
 3. An optical deviceaccording to claim 1 further including additional coupling means havinga pair of input ports and an output port coupled to one of said secondpair of ports such that said optical device may be used as an opticallogic gate.
 4. A device according to claim 1, where the second materialis relatively more dispersive than the first material.
 5. A deviceaccording to claim 1, wherein the optical waveguide is formed from asingle optical fibre.
 6. An optical device according to claim 1 whereinthe waveguide comprises material which supports soliton effects whenoptical pulses at appropriate working intensities are injected into thedevice, the length of the waveguide being sufficient such that theintensity dependent phase of an injected pulse is substantially uniformthroughout the pulse after propagation through the waveguide.
 7. Anoptical device according to claim 1 wherein the non-linear material isprovided as an overlay on-a portion of the waveguide.
 8. An opticalamplifier including an optical device according to claim 1; additionalcoupling means having a pair of input ports and an output port opticallycoupled with one of the second pair of ports of the one coupling means;and a bias signal source coupled with one of the input ports of theadditional coupling means.
 9. A method of processing an optical signalcomprising the steps of:providing an optical device having a couplingmeans having first and second pairs of optical communication ports, inwhich portions of an optical signal received at a port of one pair arecoupled into each port of the other pair in a predetermined couplingratio; and an optical waveguide at least a portion of which includes afirst material having a non-linear refractive index, and a secondmaterial in series with said first material, the first and secondmaterials having non-commuting effects on an optical signal at a workingintensity travelling along the waveguide the optical waveguide couplingtogether the first pair of ports; the coupling ratio and appropriatewaveguide parameters being selected such that in use the portions of anoptical signal at a working intensity received at one of the second pairof ports of the coupling means, and coupled into each end of thewaveguide, return to said coupling means with an intensity dependentrelative phase shift after travelling around the waveguide; andinputting an optical signal into one of the second pair of ports of thedevice to produce two counter propagating signals within the waveguide,thereby to provide a processed pulse signal output at least at one ofthe second pair of ports.
 10. A method according to claim 9 comprisinginputting an optical signal having a duration less than the transit timefor propagation around the waveguide.
 11. A method according to claim 9for processing an optical signal comprising pulses capable of solitonpropagation comprising the steps of:selecting parameters for thewaveguide of said optical device which are appropriate for solitonpropagation; inputting a pulse signal into one of the second pair ofports of the device, at an amplitude appropriate for soliton propagationin the waveguide, thereby to produce two counter propagating signalswithin the waveguide and to provide a processed pulse signal output atleast at one of the second pair of ports.
 12. An optical devicecomprising:a coupling means having first and second pairs of opticalcommunication ports, in which portions of an optical signal received ata port of one pair are coupled into each port of the other pair in apredetermined coupling ratio; and an optical waveguide comprisingmaterial which supports soliton effects when optical pulses atappropriate working intensities are injected into the device, the lengthof the waveguide being sufficient such that the intensity dependentphase of an injected pulse is substantially uniform throughout the pulseafter propagation through the waveguide, at least a portion of suchwaveguide including a first material having a non-linear refractiveindex, the optical waveguide coupling together the first pair of ports;the coupling ratio and appropriate waveguide parameters being selectedsuch that in use the portions of an optical signal at a workingintensity received at one of the second pair of ports of the couplingmeans and coupled into each end of the waveguide return with anintensity depending relative phase shift after travelling around thewaveguide.
 13. An optical amplifier including an optical devicecomprising:a coupling means having first and second pairs of opticalcommunication ports, in which portions of an optical signal received ata port of one pair are coupled into each port of the other pair in apredetermined coupling ratio; and an optical waveguide at least aportion of which includes a first material having a non-linearrefractive index, the optical waveguide coupling together the first pairof ports, the coupling ratio and appropriate waveguide parameters beingselected such that in use the portions of an optical signal at a workingintensity received at one of the second pair of ports of the couplingmeans and coupled into each end of the waveguide return with anintensity dependent relative phase shift after travelling around thewaveguide; additional coupling means having a pair of input ports and anoutput port optically coupled with one of the second pair of ports ofthe one coupling means; and a bias signal source coupled with one of theinput ports of the additional coupling means.
 14. An optical logic gateincluding an optical device comprising:a coupling means having first andsecond pairs of optical communication ports, in which portions of anoptical signal received at a port of one pair are coupled into each portof the other pair in a predetermined coupling ratio; and an opticalwaveguide at least a portion of which includes a first material having anon-linear refractive index, the optical waveguide coupling together thefirst pair of ports; the coupling ratio and appropriate waveguideparameters being selected such that in use the portions of an opticalsignal at a working intensity received at one of the second pair ofports of the coupling means and coupled into each end of the waveguidereturn with an intensity dependent relative phase after travellingaround the waveguide.